1. Field of the Invention
The invention relates to the field of dyeing of plastics. More particularly, it relates to dyeing polycarbonate with photochromic dyes.
2. The Prior Art
Attempts have been made to blend photochromic dyes into thermoplastics such as polycarbonate, which is a condensation polymer resulting typically from the reaction of bisphenol A derivatives with phosgene. However, the dyes do not survive the temperatures involved in heating the polycarbonate to form the molded article. Typically, the polycarbonate is provided in pellets suitable for gravity feed into the barrel of a molding machine.
To overcome this problem, the prior art discloses treating the polycarbonate pellets with a solution of photochromic dye in a cyclohexanone solvent and drying the treated pellets to evaporate the cyclohexanone in a first drying step. However, the pellets coalesce into clumps, entraining some cyclohexanone which cannot be driven off. We have found that any attempt to use these clumps for molding falls, since the molding material becomes a viscous mass which adheres to the mold and cannot form the desired article. The clumps must be ground to particles of a size equal to or less than that of the original pellets to expose the solvent-entraining surfaces. The ground particles are subject to a second drying step to evaporate the entrained solvent; and the dried particles are used to mold an article. While the solvent cyclohexanone enables photochromic dyes to survive during molding, the use of cyclopentanone or cycloheptanone solvents have minimal effect in providing a molded thermoplastic article with photochromic properties.
One object of our invention is to eliminate the grinding and second drying steps.
Another object of our invention is to extrude the pellets into a continuous length which is treated with a cyclohexanone dye solution and then dried in a single step.
A further object of our invention is to extrude the pellets into a plurality of discrete lengths and, while keeping their surfaces separated, treat and dry the plurality of lengths.
A still further object of our invention is to prevent the entrainment of any cyclohexanone within the thermoplastic material.
These objects are accomplished by providing a relatively large length of thermoplastic material and more particularly polycarbonate. The length is. formed either as a sheet with a small thickness and an appreciably larger width or as a rod where both cross-sectional dimensions are relatively small. The length is treated with a heated solution of photochromic dye in a cyclohexanone solvent which attacks the surface of the polycarbonate. The length is dried to evaporate the solvent and then cut into small pieces suitable for storage or introduction directly into a molding machine.
Alternatively, a plurality of lengths with at least one relatively small cross-sectional dimension, as described above, are provided. While maintaining the surfaces of the lengths separated from each other, they are treated with the heated solution and dried. The dried lengths are cut into small pieces suitable for storage or introduction directly into a molding machine.